Isolation, Characterization, and Reactivity of Co-Oxo Complexes Supported by Tris(imidazol-2-ylidene)phenylborate Ligands

C-H activation and O-O bond formation are extremely important reactions for efficient complex organic synthesis and water oxidation. The catalysts that mediate these fundamental processes are proposed to utilize transition metal-oxo intermediates, inspiring decades of research into these species. In particular, late transition metal-oxo complexes such as those with Co have been implicated in these reactions. However, due to their high d-electron counts, these complexes are difficult to isolate and characterize. Prior to the work described here, no characterizable examples of terminal Co(III)-oxo complexes were reported. Use of strongly donating ligands and low-coordinate geometries enable their isolation. Subsequent characterization of their bonding and thermodynamic properties, as well as examination of their reactivity, provides fundamental insight into the nature of these species which have often been invoked, but rarely observed, in the reactions described above. Chapter 2 describes the use of tris(imidazol-2-ylidene)phenylborate ligands to enable the isolation and characterization of three Co(III)-oxo complexes. These complexes have 6 d-electrons, yet maintain a Co-O multiple bond. All have been crystallographically characterized and display H-atom and O-atom transfer reactivity. Characterization of the thermodynamic properties of the two terminal oxo complexes reveals ligand substituent-dependent oxidation potentials and pKa values. Interestingly, the Co-O multiple bond itself is unaffected by the ligand substituents and DFT analysis reveals the presence of two strong π bonds and one weak σ bond. The C-H activation reactivity of the tBu-substituted Co(III)-oxo complex is described in Chapter 3. Broadly, transition metal-oxo complexes react quickest with the homolytically weakest C-H bond in a molecule, resulting in selectivity based on bond strengths. However, thorough kinetic studies reveal that this Co(III)-oxo complex reacts quickest with the most acidic C-H bond, opening up possibilities of alternative selectivity. It is shown that these C-H activation reactions are concerted but proceed through an imbalanced transition state (“asynchronous” CPET) which had only been explored computationally previously. Chapter 4 describes the C-H and O-H activation reactivity of the Ad-substituted Co(III)-oxo complex which is more basic and less oxidizing than the tBu-substituted complex. This results in C-H activation that proceeds with greater asychronicity, seen in Hammett analysis. Additionally, a mechanistic switch from asynchronous CPET to stepwise PTET is observed for O-H activation. This highlights the limits of asynchronous CPET and provides additional support for the concerted reactivity invoked in Chapter 3. Most isolable transition metal-oxo complexes engage in endergonic C-H activation reactions at slow rates. This is true for the Co(III)-oxo complexes described here, prompting attempts at generating a more active Co(IV) oxidant. These experiments are summarized in Chapter 5, revealing ligand substituent-dependent reactivity. For the tBu-substituted Co(III)-oxo, preliminary evidence indicates likely oxo-oxo coupling to form a Co(III)2-peroxo complex. For the Ad-substituted Co(III)-oxo, oxidation results in the formation of a transient Co(IV)-oxo complex, as assayed by EPR spectroscopy. This intermediate reacts endergonically with a ligand C-H at rates comparable to P450 enzymes, before engaging in exergonic rebound to form hydroxylated product. This reactivity pattern mirrors that proposed to be operative in enzymatic and catalytic systems, providing support for these mechanisms